1. Technical Field
The present invention relates to a method for producing a rubber composition and a method for manufacturing a pneumatic tire, and further relates to a rubber composition and a pneumatic tire obtained by the respective methods.
2. Related Art
Styrene-butadiene rubber having large styrene unit content is conventionally used in a rubber composition used in, for example, a pneumatic tire in order to improve grip performance (wet grip performance) on a wet road surface. However, use of the styrene-butadiene rubber having high styrene content involves deterioration of low temperature performance, abrasion resistance, low heat generation performance (low rolling resistance) and the like.
Furthermore, it is known to add syndiotactic-1,2-polybutadiene (hereinafter referred to as “SPB”) to a silica-containing rubber composition using the silica as a filler. For example, JP-A-07-188461 discloses that SPB having a melting point of from 120 to 180° C. is added to diene rubber together with silica for the purpose of improving low rolling resistance and processability without deteriorating abrasion resistance, fracture resistance and wet grip performance.
JP-A-2001-233994 discloses that a polymeric compound having a melting point of from 80 to 230° C. is added to diene rubber in order to improve the performance of ice controlling action while maintaining wet grip performance, and SPB is exemplified as the polymeric compound.
JP-A-2009-235191 discloses to add SPB to modified styrene-butadiene rubber having a functional group in order to improve processability while maintaining the balance among abrasion resistance, fracture characteristics, low heat generation performance and wet grip performance.
JP-A-2014-084410 discloses that to improve low rolling resistance without deteriorating abrasion resistance, diene rubber is kneaded with SPB having a melting point of 110° C. or lower to prepare a dry master batch and silica is then added to the dry master batch, followed by kneading.
In those prior arts, SPB is added to diene rubber in the same kneading step as silica, or SPB is previously kneaded with diene rubber, and silica is then added to the kneaded mixture obtained, followed by kneading.
It is generally considered that a resin having a melting point higher than room temperature has the effect of improving wet grip performance. However, due to that SPB is compatible with silica, it is difficult to sufficiently bring out the performance of SPB in the above-described prior arts, and the effect of improving wet grip performance is poor.